Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Mar. 31, 2014
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	$ 81		$ 55
Liabilities	(110)		(79)
Derivative Asset, Master Netting Adjustment	(33)	[1]	(20)	[1]
Derivative Liability, Master Netting Adjustment	33	[1]	20	[1]
Derivative Asset	48		35
Derivative Liability	(77)		(59)
Level 2 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	81		55
Liabilities	(95)		(60)
Level 2 Instruments [Member] | Aluminum Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	40		23
Liabilities	(44)		(36)
Level 2 Instruments [Member] | Currency Exchange Contract [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	40		28
Liabilities	(49)		(23)
Level 2 Instruments [Member] | Interest Rate Swaps [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	(1)		0
Level 2 Instruments [Member] | Energy Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	1		4
Liabilities	(1)		(1)
Level 3 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	(15)		(19)
Level 3 Instruments [Member] | Energy Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	$ (15)		$ (19)

[1] Amounts represent the impact of legally enforceable master netting agreements that allow the Company to settle positive and negative positions with the same counterparties.